Microlithography (also called photolithography or simply lithography) is a technology for the fabrication of integrated circuits, liquid crystal displays and other microstructured devices. The process of microlithography, in conjunction with the process of etching, is used to pattern features in thin film stacks that have been formed on a substrate, for example a silicon wafer. In general, at each layer of the fabrication, the wafer is first coated with a photoresist which is a material that is sensitive to radiation, such as deep ultraviolet (DUV) light. Next, the wafer with the photoresist on top is exposed to projection light in a projection exposure apparatus. The apparatus projects a mask containing a pattern onto the photoresist so that the latter is only exposed at certain locations which are determined by the mask pattern. After the exposure the photoresist is developed to produce an image corresponding to the mask pattern. Then an etch process transfers the pattern into the thin film stacks on the wafer. Finally, the photoresist is removed. Repetition of this process with different masks results in a multi-layered microstructured component.
A projection exposure apparatus typically includes an illumination system for illuminating the mask, a mask stage for aligning the mask, a projection objective and a wafer alignment stage for aligning the wafer coated with the photoresist. The illumination system illuminates a field on the mask that may have the shape of a rectangular or curved slit, for example.
In current projection exposure apparatus a distinction can be made between two different types of apparatus. In one type each target portion on the wafer is irradiated by exposing the entire mask pattern onto the target portion in one go. Such an apparatus is commonly referred to as a wafer stepper. In the other type of apparatus, which is commonly referred to as a step-and-scan apparatus or scanner, each target portion is irradiated by progressively scanning the mask pattern under the projection beam along a scan direction while synchronously moving the substrate parallel or anti-parallel to this direction.
It is to be understood that the term “mask” (or reticle) is to be interpreted broadly as a patterning mechanism. Commonly used masks contain transmissive or reflective patterns and may be of the binary, alternating phase-shift, attenuated phase-shift or various hybrid mask type, for example. However, there are also active masks, e.g. masks realized as a programmable mirror array. Also programmable LCD arrays may be used as active masks.
As the technology for manufacturing microstructured devices advances, there are ever increasing demands also on the illumination system. Ideally, the illumination system illuminates each point of the illuminated field on the mask with projection light having a well defined irradiance and angular distribution. The term angular distribution describes how the total light energy of a light bundle, which converges towards a particular point in the mask plane, is distributed among the various directions of the rays that constitute the light bundle.
The angular distribution of the projection light impinging on the mask is usually adapted to the kind of pattern to be projected onto the photoresist. For example, relatively large sized features may involve a different angular distribution than small sized features. The most commonly used angular distributions of projection light are referred to as conventional, annular, dipole and quadrupole illumination settings. These terms refer to the irradiance distribution in a pupil plane of the illumination system. With an annular illumination setting, for example, only an annular region is illuminated in the pupil plane. Thus there is only a small range of angles present in the angular distribution of the projection light, which means that all light rays impinge obliquely with similar angles onto the mask.
Different ways are known for modifying the angular irradiance distribution of the projection light in the mask plane so as to achieve the desired illumination setting. In the simplest case a stop (diaphragm) including one or more apertures is positioned in a pupil plane of the illumination system. Since locations in a pupil plane translate into angles in a Fourier related field plane such as the mask plane, the size, shape and location of the aperture(s) in the pupil plane are involved in determining the angular distributions in the mask plane. However, any change of the illumination setting involves a replacement of the stop. This can make it difficult to finely adjust the illumination setting, because this would typically involve a very large number of stops that have aperture(s) with slightly different sizes, shapes or locations. Furthermore, the use of stops inevitably results in light losses and thus reduces the throughput of the apparatus.
Light losses caused by stops are avoided if diffractive optical elements are used to produce a specific irradiance distribution in the pupil plane of the illumination system. The irradiance distribution can be modified, at least to a certain extent, by adjustable optical elements such as zoom lenses or a pair of axicon elements that are arranged between the diffractive optical element and the pupil plane.
Flexibility in producing different irradiance distributions in the pupil plane can be increased by using mirror arrays instead of the diffractive optical elements. For example, EP 1 262 836 A1 proposes the use of a mirror array that is realized as a microelectromechanical system (MEMS) including more than 1000 microscopic mirrors. Each of the mirrors can be tilted in two different planes perpendicular to each other. Thus radiation incident on such a mirror device can be reflected into (substantially) any desired direction of a hemisphere. A condenser lens arranged between the mirror array and the pupil plane translates the reflection angles produced by the mirrors into locations in the pupil plane. This prior art illumination system makes it possible to illuminate the pupil plane with a plurality of spots, wherein each spot is associated with one particular microscopic mirror and is freely movable across the pupil plane by tilting this mirror.
Similar illumination systems are known from US 2006/0087634 A1, U.S. Pat. No. 7,061,582 B2, WO 2005/026843 A2 and WO 2010/006687.
However, in general, the use of mirror arrays is technologically demanding and involves sophisticated optical, mechanical and computational solutions.
A simpler approach to produce continuously variable irradiance distributions in the pupil plane is the use of diffractive optical elements having position dependent diffractive effects. Depending on the position where the projection light impinges on the element, different irradiance distributions are produced in the pupil plane. Usually the projection light beam will be kept fixed and the diffractive optical element is displaced with the help of a displacement mechanism so as to change the position where the projection light beam impinges on the element. Diffractive optical elements of this kind are commercially available from Tessera Technologies, Inc., San Jose, USA.
However, the flexibility to produce different irradiance distributions in the pupil plane can be quite restricted with such diffractive optical elements. Generally, at most there are two available degrees of freedom that can be used to modify this irradiance distribution, namely moving the diffractive optical element along one direction and moving it along an orthogonal direction. Displacing the diffractive optical element along the optical axis generally has very little effect on the irradiance distribution.